Photonic band gap fibers are known as one of optical fibers used for optical communication or in fiber laser devices. Further, as one of photonic band gap fibers, there are photonic band gap fibers provided with a plurality of high refractive index portions including a material having a high refractive index than a core in a clad which coats the core, and the photonic band gap fibers are used as optical filters or polarization maintaining optical fibers.
The following Patent Document 1 discloses an example of such photonic band gap fibers. The photonic band gap fiber disclosed in the Patent Document 1 has a core with which a rare earth element is doped and a clad which coats the core, and, in the clad, periodic structure areas in which a plurality of high refractive index portions are aligned in a triangular grid pattern are formed to sandwich the core. According to Patent Document 1, the periodic structure areas can restrict propagation of a wavelength which does not need to be amplified among spontaneous emission light emitted from the rare earth element.
Further, the following Patent Document 2 discloses a method of manufacturing a photonic band gap fiber. With the method of manufacturing a photonic band gap fiber, first, a glass rod which becomes a core with which a rare earth element is doped, bilayer glass rods in which high refractive index portions having a higher refractive index than silica are coated by an outer layer made of silica, and a silica tube which is made of silica are prepared. Next, in a penetration hole of the silica tube, a glass rod which becomes a core is arranged in the center of the penetration hole, and the bilayer glass rods are arranged around the core. Next, the silica tube in which the glass rod which becomes a core and the bilayer glass rods are arranged is heated in vacuum to fill the gap in the penetration hole in the silica tube and manufacture a photonic band gap fiber base material. Further, the photonic band gap fiber base material is drawn to obtain a photonic band gap fiber.